Mybatis源码学习笔记之Mybatis一级缓存

  一级缓存其实就是会话级的缓存，默认开启(可通过配置flushCache= true关闭)，指的是在同一会话内如果有两次相同的查询（Sql和参数均相同），那么第二次就会命中缓存。

  一级缓存命中条件之一是CacheKey相同：
  (1)SQL及参数相同
  (2)同一个会话
  (3)相同的MapperStatement ID
  (4)查询结果集的分页信息

  为了方便理解，这里贴出Cachkey的生成代码（源码位置：org.apache.ibatis.executor.baseExecutor）

  @Override
  public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    CacheKey cacheKey = new CacheKey();
    cacheKey.update(ms.getId());
    cacheKey.update(rowBounds.getOffset());
    cacheKey.update(rowBounds.getLimit());
    cacheKey.update(boundSql.getSql());
    List parameterMappings = boundSql.getParameterMappings();
    TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
    // mimic DefaultParameterHandler logic
    for (ParameterMapping parameterMapping : parameterMappings) {
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        Object value;
        String propertyName = parameterMapping.getProperty();
        if (boundSql.hasAdditionalParameter(propertyName)) {
          value = boundSql.getAdditionalParameter(propertyName);
        } else if (parameterObject == null) {
          value = null;
        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
          value = parameterObject;
        } else {
          metaObject metaObject = configuration.newmetaObject(parameterObject);
          value = metaObject.getValue(propertyName);
        }
        cacheKey.update(value);
      }
    }
    if (configuration.getEnvironment() != null) {
      // issue #176
      cacheKey.update(configuration.getEnvironment().getId());
    }
    return cacheKey;
  }

  (1)手动调用clearCache
  (2)执行提交回滚
  (3)执行update
  (4)配置flushCache=true
  (5)缓存作用域为Statement

   源码位置：org.apache.ibatis.executor.baseExecutor

@SuppressWarnings("unchecked")
  @Override
  public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) { //判断flushCache参数是否开启
      clearLocalCache();
    }
    List list;
    try {
      queryStack++;
      list = resultHandler == null ? (List) localCache.getObject(key) : null;//查询缓存
      if (list != null) { //有缓存读缓存
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else { //无缓存走查询数据库
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

   这里贴出无缓存mybatis（需要查询数据库同时设置缓存）情形的处理。

private  List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }